One promising alternative to fossil fuels is hydrogen. Through its reaction with oxygen, hydrogen releases energy explosively in heat engines or quietly in fuel cells to produce water as its only byproduct. Hydrogen is abundant and generously distributed throughout the world without regard for national boundaries. Storing hydrogen in a high-energy-density form that flexibly links its production and eventual use is a key element of the hydrogen economy.
Boddien et al. (in: CO2-“Neutral” Hydrogen Storage Based on Bicarbonates and Formates. Angew. Chem. Int. Ed., 2011 50: 6411-6414) describe a ruthenium catalyst generated in situ that facilitates the selective hydrogenation of bicarbonates and carbonates, as well as CO2 and base, to give formates and also the selective dehydrogenation of formates back to bicarbonates. The two reactions can be coupled, leading to a reversible hydrogen-storage system.
KR 2004/0009875 describes an electrochemical preparation method of formic acid using carbon dioxide, thereby simultaneously carrying out reduction of carbon dioxide and conversion of carbon dioxide into useful organic matters. The method comprises electrochemical reduction of carbon dioxide using formic acid dehydrogenase or formic acid dehydrogenase-producing anaerobic bacteria and an electron carrier in which reversible oxidation/reduction is occurred at electric potential of −400 to −600 mV, wherein the concentration of the electron carrier is 5 to 15 mM; the anaerobic bacteria are selected from Clostridium thermoaceticum, Clostridium thermoauthotrophicum, Acetobacterium woodii, Acetogenium kivui, Clostridium aceticum, Clostridium ljungdahlii, Eubacterium limosum or a mixture thereof; the electron carrier is selected from methylviologen, N,N,-diethyl-4,4-bipyridyl, N,N-diisopropylyl-4,4-bipyridyl, 4,4-bipyridyl or a mixture thereof; the reduction temperature is 20 to 70° C.; and the reduction pH is 6.0 to 7.0.
WO 2011/087380 describes methods for improving the efficiency of carbon capture in microbial fermentation of a gaseous substrate comprising CO and/or H2; said method comprising applying an electrical potential across the fermentation. It further relates to improving the efficiency of carbon capture in the microbial fermentation of gaseous substrate comprising CO and/or H2 to produce alcohol(s) and/or acid (s).
Catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. Microorganisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of microorganisms to grow on CO as a sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Wood-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source, all such organisms produce at least two of these end products. The Wood-Ljungdahl pathway of anaerobic CO2 fixation with hydrogen as reductant is considered a candidate for the first life-sustaining pathway on earth because it combines carbon dioxide fixation with the synthesis of ATP via a chemiosmotic mechanism.
Schuchmann et al. (A bacterial electron-bifurcating hydrogenase. J Biol Chem. 2012 Sep. 7; 287(37):31165-71) describe a multimeric [FeFe]-hydrogenase from A. woodii containing four subunits (HydABCD) catalyzing hydrogen-based ferredoxin reduction. Apparently, the multimeric hydrogenase of A. woodii is a soluble energy-converting hydrogenase that uses electron bifurcation to drive the endergonic ferredoxin reduction by coupling it to the exergonic NAD+ reduction.
Schiel-Bengelsdorf, and Dürre (in:Pathway engineering and synthetic biology using acetogens, FEBS Letters, 2012, 586, 15, 2191) describe acetogenic anaerobic bacteria that synthesize acetyl-CoA from CO2 or CO. Their autotrophic mode of metabolism offers the biotechnological chance to combine use of abundantly available substrates with reduction of greenhouse gases. Several companies have already established pilot and demonstration plants for converting waste gases into ethanol, an important biofuel and a natural product of many acetogens. Recombinant DNA approaches now opened the door to construct acetogens, synthesizing important industrial bulk chemicals and biofuels such as acetone and butanol. Thus, novel microbial production platforms are available that no longer compete with nutritional feedstocks.
WO 2011/028137 describes a bioreactor system for fermentation of a gaseous substrate comprising CO and optionally H2, or CO2 and H2, to one or more products, including acid(s) and/or alcohol(s).
U.S. Pat. No. 7,803,589 describes an Escherichia coli microorganism, comprising a genetic modification, wherein said genetic modification comprises transformation of said microorganism with exogenous bacterial nucleic acid molecules encoding the proteins cobalamide corrinoid/iron-sulfur protein, methyltransferase, carbon monoxide dehydrogenase, acetyl-CoA synthase, acetyl-CoA synthase disulfide reductase and hydrogenase, whereby expression of said proteins increases the efficiency of producing acetyl-CoA from CO2, CO or H2, or a combination thereof.
Poehlein et al. (An ancient pathway combining carbon dioxide fixation with the generation and utilization of a sodium ion gradient for ATP synthesis. PLoS One. 2012; 7(3): e33439. doi:10.1371/journal.pone.0033439) describes that the synthesis of acetate from carbon dioxide and molecular hydrogen is considered to be the first carbon assimilation pathway on earth. It combines carbon dioxide fixation into acetyl-CoA with the production of ATP via an energized cell membrane. How the pathway is coupled with the net synthesis of ATP has been an enigma. The anaerobic, acetogenic bacterium Acetobacterium woodii uses an ancient version of this pathway without cytochromes and quinones. It generates a sodium ion potential across the cell membrane by the sodium-motive ferredoxin:NAD oxidoreductase (Rnf). The genome sequence of A. woodii solves the enigma: it uncovers Rnf as the only ion-motive enzyme coupled to the pathway and unravels a metabolism designed to produce reduced ferredoxin and overcome energetic barriers by virtue of electron-bifurcating, soluble enzymes.